The invention relates to a method for charging energy storage cells of an energy storage device and to a rechargeable energy storage device, in particular an energy storage device comprising a modular battery system for an electrically operated vehicle.
It is becoming apparent, that, in the future, increasingly electronic systems which combine new energy storage technologies with electric drive technology will be used both in stationary applications, such as wind turbines or solar systems, for example, and in vehicles, such as hybrid or electric vehicles.
The feed of polyphase current into an electric machine is conventionally effected by a converter in the form of a pulse-controlled inverter. For this purpose, a DC voltage provided by a DC link can be converted into a polyphase AC voltage, for example, a three-phase AC voltage. The DC link is in this case fed from a string of series-connected battery modules. In order to be able to meet the requirements set for a specific application in respect of power and energy, often a plurality of battery modules are connected in series in a traction battery.
The documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose battery cells which are interconnected in modular fashion in energy storage devices, which battery cells can be coupled or decoupled selectively into or from the string comprising series-connected battery cells via suitable actuation of coupling units. Systems of this type are known by the term battery direct converter (BDC). Such systems include DC sources in an energy storage module string which are connectable to a DC link for the supply of electrical energy to an electric machine or to an electric grid via a pulse-controlled inverter.
The energy storage module string in this case has a plurality of energy storage modules connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which makes it possible to bypass the respectively assigned at least one battery cell or to switch the respectively assigned at least one battery cell into the respective energy storage module string, depending on control signals. Optionally, the coupling unit can be configured such that it additionally makes it possible to switch the respectively assigned at least one battery cell into the respective energy storage module string even with inverse polarity or else to interrupt the respective energy storage module string.
BDCs generally have greater efficiency and greater failsafety than conventional systems. The failsafety is ensured, inter alia, by virtue of the fact that battery cells that are defective, have failed or are not fully functional can be disconnected from the energy supply string by suitable bypass actuation of the coupling units. The total output voltage of the energy storage module string can be varied and in particular adjusted stepwise by corresponding actuation of the coupling units. The stepping of the output voltage in this case results from the voltage of a single energy storage module, wherein the maximum possible total output voltage is determined by the sum of the voltages of all energy storage modules of the energy storage module string.
In order to adjust an output voltage of an energy storage module, pulse-width-modulated (PWM) actuation of the coupling units can take place. It is thus possible to output a desired average value as energy storage module voltage by targeted variation of the switch-on or switch-off times.
For BDCs, it is necessary to develop charging strategies with which the battery cells of the energy storage modules can be recharged simply, efficiently and flexibly.